THIS invention relates to rigid three-dimensional components, which have high rigidity and low mass, and in particular to coated speaker domes.
There are many applications requiring structures of high rigidity and low mass. Typical applications are in the aerospace industry where virtually all mechanical components must have a high rigidity to mass ratio.
However, there is a range of other applications for light but rigid bodies. A particular application is the production of drive units for acoustic loudspeakers, and in particular high frequency tweeters for the accurate reproduction of high frequency sounds.
Human hearing is commonly accepted to cover the range 20 Hz-20 kHz. Therefore a high quality loudspeaker system needs to accurately reproduce frequencies at least over this frequency range. Typical high performance loudspeakers employ two or more drive units that are effectively mechanical transducers converting an electrical signal into a sound (compression) wave. Each drive unit will cover a specific part of the audible range. The drive unit can be approximated to a piston moving backwards and forwards to create compression and rarefaction of air.
It is well known that small pistons can efficiently generate high sound pressure levels at high frequencies while larger diameter pistons are required to produce comparable sound pressure levels at lower frequencies with comparable efficiency. Typically a 25 mm diameter drive unit can operate in the frequency range 2-20 kHz while a larger drive unit of, say, 100-250 mm diameter can produce frequencies in the range down to 100 Hz and below. However, larger drive units cannot easily be used to produce high frequency sounds due to the problems of unwanted oscillations or break-up that can occur. Human ears are very sensitive to the colouration of the sound by these break-up modes. For this reason high frequency drive units generally have a small diameter. Recently it has been demonstrated that the presence of break-up modes at frequencies that lie outside the accepted range of human hearing can cause audible degradation of the source. For this reason several attempts have been made to produce drive units that can operate at frequencies higher than 20 kHz without distortion.
The ideal loudspeaker would have very low mass, to enhance its sensitivity, and very high rigidity with no resonances within or close to the frequency spectrum of operation which could affect the audible output. All practical tweeter devices naturally have mass, and also resonances. Developments in audio media and amplification systems, such as the so called Super Audio formats (SACD and DVDA) extend the range of frequencies provided in the drive to modern speakers up to as high as 96 kHz, compared for example with the upper limit of the bandwidth of a standard CD, which is about 22 kHz.
It is well known that lighter and more rigid tweeter structures, fabricated using materials with a higher value of Young's modulus and lower density, show higher frequency resonances. As such, the use of diamond in tweeters is well reported. Prior art records a variety of configurations of speaker dome, fabricated by a range of means, but the performance advantage reported is generally poor and such speaker domes are not in widespread use. There is also substantial prior art in tweeter devices based on other materials such as Al, Be and plastics, and on a range of geometries.
U.S. Pat. No. 5,556,464 discloses the use of diamond domes for speakers, describing in detail the need to terminate the edge of the integral planar flange in a manner designed to control edge cracks developing. DE Patent 10049744 discloses the use of a diamond dome mounted concave onto a voice coil former, such that the edges of the dome are unsupported. This type of geometry provides for a range of unwanted resonances in the dome structure that may colour the output sound. More recently, Bower and Wilkins (B&W Loudspeakers Ltd, Dale Road, Worthing, West Sussex, England) have launched a range of speakers using diamond domes, the design of which is described in co-pending GB patent application 0408458.8.
However there are limitations on the use of diamond and other stiff materials in speaker domes, particularly in the larger units required for large auditoriums, for example. The resonance frequencies of such larger units cannot easily be displaced to high frequencies beyond the point at which they impact on the audible perception, and the nature of high stiffness materials and high rigidity structures is generally to have low damping or a high Q factor at resonance.